Lasers are commonly used today for a variety of medical applications. One of these applications is to perform dermatological procedures, such as removal of foreign pigments in the skin (tattoos), treatment of vascular disorders which cause unwanted pigmentation ("wine marks"), and to destroy skin tumors by ablating the affected tissue.
In most dermatological applications, a handpiece is used to guide the output of the laser to the patient's skin and to form a laser spot of a desired size on the region of the skin which is to be treated. The handpiece is typically attached to one end of an articulated arm which transmits the output of a medical laser (such as a CO.sub.2 laser for many dermatological applications) to the handpiece and allows the handpiece to have a wide range of motion.
Prior art dermatological handpieces rely on a parfocal optical system to bring the laser beam to a focus. One such optical system is a zoom lens system which is contained in a dermatology handpiece for Argon lasers manufactured by Nidek of Tokyo, Japan. This zoom system is coupled to an optical fiber at one end and to an Argon laser at the other end. In such a system an image of the end of the optical fiber is relayed to the work area, thereby producing a well defined laser spot. Coherent, Inc. of Palo, Alto, Calif., the assignee of the present invention, produces a dermatology handpiece for use with CO.sub.2 lasers. This handpiece is not coupled to an optical fiber because present fibers do not have sufficient transmissivity at the wavelength of the laser beam produced by CO.sub.2 lasers. The laser spot produced by this handpiece has an intensity which decreases as a function of the distance from the beam center, owing to the Gaussian nature of the output beam.
It is desireable that the laser beam be in focus at the skin surface so that little heat transfer or ablation occurs outside of the desired treatment area and at locations beneath the skin surface. It is also desireable that the beam be highly collimated, that is composed of substantially parallel rays of light, so that the overall laser beam diverges very little over a working region in front of the skin surface. This characteristic reduces collateral damage to the areas surrounding the work area and makes it easier for the surgeon to perform the procedure since the size of the laser spot is less sensitive to small movements of the handpiece.
The energy density of a pulsed laser beam which is delivered to a patient's tissue depends on the area of the beam at the tissue surface and the energy content of the pulse. The area of the laser beam at the tissue surface is proportional to the square of the spot size, which is determined by the spot diameter. Thus, if the spot size changes as the surgeon moves the handpiece over a range of working positions, the energy density can be changed to an even greater degree. This can lead to difficulties in properly completing the procedure. A highly collimated beam thus insures that under practical working conditions, the energy density of the laser beam delivered to the working area is determined by the surgeon's selection of the pulse energy for the laser pulses and not by the movements of the handpiece.
Prior art handpieces such as the Nidek system referred to above are capable of delivering a beam having a range of spot sizes, however, each spot size is in focus within only a very limited range of distances from the end of the handpiece. Thus, the laser beam produced by this system is not highly collimated. Similarly, the beam produced by the Coherent handpiece is also not highly collimated. This feature means that the surgeon has to maintain an approximately constant distance between the end of the handpiece and the target tissue in order to safely and effectively perform the procedure.
FIG. 1 shows the primary components of a prior art laser system suitable 10 for delivering a laser spot to a desired region of a patient's skin. Laser system 10 includes a laser source 12 which produces a laser beam at its output and an articulated arm 14 which relays the beam to a handpiece 16, through which a beam 11 exits and propagates to the surface of a patient's skin.
Articulated arm 14 is composed of multiple successive straight segments 15 interconnected by rotatable joints 17. Each rotatable joint 17 is associated with a mirror 19. A laser beam entering each segment 15 of articulated arm 14 is redirected down the subsequent segment 15 by the mirror 19 associated with each rotatable joint 17. The rotatable joints 17 are able to rotate while maintaining beam alignment down each successive segment 15 so that delivery segment 21 can direct the beam to a target location at a desired angle. Delivery segment 21 directs the beam out of articulated arm 14 to either the target tissue or another delivery system, such as handpiece 16. Handpiece 16 usually contains an optical system 18 which acts to focus the beam, producing beam 11 which propagates to the patient's skin.
In addition to the mentioned elements, laser system 10 also usually includes a pointing laser 13 which produces a pointing spot which is co-linear with the output beam of laser 12 after its propagation through arm 14 and handpiece 16. The pointing spot assists the surgeon to correctly position the medical laser beam on the patient's skin.
Optical system 18 is capable of delivering laser energy in the form of a clearly focused spot to the surface of a patient's skin. In some systems, the spot size may be varied in diameter, however in typical prior art systems, the spot size selected will remain in focus only if handpiece 16 is confined to motion over a very limited range of distances around the position at which the spot is properly focused. This requires the surgeon performing the procedure to maintain a constant (or nearly constant) distance between the end of the handpiece and the skin surface. Under actual operational conditions this requirement is sometimes difficult to satisfy and places an additional constraint on the surgeon which can impact their ability to effectively carry out the procedure.
What is desired is a laser handpiece for use in dermatological applications which is capable of delivering a well defined, focused laser spot to the surface of a patient's skin, where the spot size is largely insensitive to movement of the handpiece over a range of working positions. It is also desired to have a handpiece which is capable of producing a variety of spot sizes, where each selected spot size is largely insensitive to movement of the handpiece over a range of working positions.